The present invention relates to a focus adjusting mechanism for use in a device having variable magnification means, and more particularly to a focus adjusting mechanism for use with variable magnifiction means such as a plurality of projecting lenses of varying focal lengths, and adapted for use in reader-printers and like apparatus.
Microfiche films, one type of microfilms, are used at various reduced scales (microfilming reduction ratios) generally for recording literature, papers and documents. The recorded images carried on microfiche films are viewed or copied with the use of reader-printers which, if equipped with a single projecting lens of definite focal length, will involve a limitation on the size of projectable enlarged images.
Accordingly, conventional reader-printers may be provided with a plurality of projecting lenses of varying focal lengths, one of which will be selectively positioned in an operative position to give the desired magnification in accordance with a particular microfiche film. These lenses, however, require a space for storage, are not easily and quickly changed and must be focused every time the lenses are changed.
To overcome these drawbacks, an apparatus has been proposed and placed into use as disclosed in U.S. Pat. No. 3,713,737. The disclosed apparatus comprises a plurality of projecting lenses having varying focal lengths and a support plate carrying the lenses together and movable to bring the desired projecting lens to a projecting position. With the movement of the support plate, the focusing ring on the selected lens comes into engagement with a single focusing dial fixedly provided in the vicinity of the projecting position so that the lens can be focused by the dial. Although the apparatus has found wide use because of its outstanding usefulness, the apparatus still remains to be improved in respect of the following drawback.
With the apparatus described, the change of the projecting lenses involves disengagement and engagement between the focusing ring of the lens and the focusing dial. This could permit a small amount of rotation of the focusing ring entirely independently of focusing adjustment. This phenomenon will be described below in greater detail. When the focusing ring on a lens moves with the support plate relative to the focusing dial, the ring has not been completely disengaged from the dial means in the initial stage of the relative movement, with the result that this movement entails slight rotation of the ring and the dial means. This phenomenon also takes place when the focusing ring in a disengaged position comes into engagement with the dial means. This gives rise to a serious objection, for example, when a projecting lens, once focused, is shifted to a non-projecting position and thereafter returned to the projecting position again, because each shift of the lens to the non-projecting position and then back to the projecting position causes rotation of the focusing ring on the lens, thereby bringing the lens out of focus and necessitating focus adjustment again.
To overcome the above drawback, Published Unexamined Japanese Utility Model Application No. 51/52252 proposes an expedient in which the objectionable rotation of the focusing ring of the lens is precluded by rendering the focus adjusting dial means less resistant to rotation than the focusing ring. Although the objectionable rotation of the focusing ring can be prevented to some extent according to the proposal, the drawback still remains to be fully eliminated, consequently entailing the frequent necessity of refocusing during use.
For reference, the focus adjustment of projecting lenses for reader-printers will be described specifically with numerical values. The projecting lenses usually used have a magnification of about 10 to about 50X. Thus, even the slightest shift in the focus of the lens will produce blurred enlarged images upon projection. It has been found that the projecting lens, when shifted about 0.02 mm from its focused position, produces blurred images which can be detected with the unaided eye. Calculated as the angular displacement of the focusing ring of the lens, such amount of shift corresponds to as small as about 3 to about 4 deg.